for Photographic Drawing. 171 



go along with the bichromate of potash, the coloiu' of the ob- 

 ject and of the paper being then of different shades of green. 

 In this way also the object may be represented of a darker 

 shade than the ground. 



Paper prepared mth bichromate of potash is equally sensi- 

 tive with most of the papers prepared with salts of silver, 

 though inferior to some of them. It is not sufficiently sensi- 

 tivefor the camera obscura, but answers quite well for taking 

 drawings from dried plants, or for copying prints, &c. Its 

 great recommendation is its cheapness and the facility with 

 which it can be prepared. The price of the bichromate 

 of potash is 2s. 6d. per lb., whereas of the nitrate of silver 

 only half an ounce can be obtained for that sum. The pre- 

 paring of paper TNdth the salts of silver is a work of extreme 

 nicety, whereas both the preparing of the paper with the bi- 

 chromate of potash and the subsequent fixing of the images 

 are matters of great simphcity, and I am therefore hopeful 

 that this method may be found of considerable practical uti- 

 lity in aiding the operations of the lithographer. 



M. Ponton. 



Edinhirgh, 18{/i May 1839. 



Address to the Geological Society of London., delivered at the 

 Anniversary, on the 15th of February 1839. By the Rev. 

 William Whewell, B.D., F.R.S., President of the So- 

 ciety.* 



" Gentlemen, — In attempting a sketch of the subjects which 

 have occupied the attention of the Society dm'ing the year, I 

 should wish to retain that distribution of the science of geology 

 according to which I arranged my remarks in the Address which 

 I had last year the honour of reading to the Society ; I mean 

 tlie primary division into Descriptive Geology and Geological 

 Dynamics ; the former implying a description of the rocks of 

 tlie earth's surface according to an established classification of 



* The l)iofji"iphical part of this Address, want of room forces us to delay 

 till another opportunity. 



